H1N1 "swine flu" was first identified in Mexico in the spring of 2009 and soon became a worldwide epidemic. The U.S. Department of Health and Human Services declared a public health emergency in April, and the World Health Organization (WHO) raised the worldwide pandemic alert level to Phase 6 -- its highest level -- in June.

The 2009 H1N1 strain was found to actually be a combination of swine, avian, and human influenza strains. It spread easily from person-to-person, and led to unusually severe disease because most of the population did not have pre-existing immunity. After early shortages, an effective H1N1 vaccine was widely distributed and the WHO declared an end to the pandemic in August 2010.

WHO received reports of 18,500 laboratory-confirmed deaths worldwide caused by H1N1 pandemic influenza A between April 2009 and August 2010. Because reporting on a global scale generally under-counts the real number of cases, Fatima Dawood from the Centers for Disease Control and Prevention and colleagues sought to estimate the actual number of deaths during the first 12 months of virus circulation in each country.

The researchers calculated crude respiratory mortality rates associated with the 2009 H1N1 pandemic by age (0-17, 18-64, and over 64 years) using cumulative numbers of symptomatic attacks associated with the virus in 12 countries and symptomatic case fatality ratios (how many symptomatic people died) from 5 high-income countries with good reporting. These ratios were not available for many lower-income countries in Asia and Africa.

To adjust raw respiratory mortality rates for differences among countries in risk of death from flu, they developed a "respiratory mortality multiplier." They also calculated cardiovascular disease mortality rates associated with the flu pandemic as the ratio of excess deaths from cardiovascular and respiratory diseases during the pandemic period in 5 countries, multiplied by the crude flu-associated respiratory disease mortality rate. These respiratory and cardiovascular mortality rates were then multiplied by age to calculate the total number of H1N1 flu-associated deaths.

Results

The researchers estimate that there were 201,200 H1N1-related respiratory deaths, with a range of 105,700 to 395,600.

There were also an estimated additional 83,300 cardiovascular deaths associated with the pandemic, with a range of 46,000 to 179,900.

80% of these respiratory and cardiovascular deaths were of people younger than 65 years.

Approximately half of these estimated H1N1 flu deaths occurred in Southeast Asia and Africa, versus only about 12% of officially reported deaths.

"Our estimate of respiratory and cardiovascular mortality associated with the 2009 pandemic influenza A H1N1 was 15 times higher than reported laboratory-confirmed deaths," the study authors concluded.

"Although no estimates of [symptomatic case fatality ratios] were available from Africa and southeast Asia, a disproportionate number of estimated pandemic deaths might have occurred in these regions," they added. "Therefore, efforts to prevent influenza need to effectively target these regions in future pandemics."